In a lithography process employed for producing a semiconductor, formation of a finer pattern is increasingly required so as to match with an elevated degree of integration. For formation of a finer pattern, it is essential that the length of the light wave beaming from an exposure light source be made as shorter as possible. At present, a lithography using a krypton fluoride (KrF) excimer laser (wavelength: 248 nm) is becoming mainstream, and a lithography using an argon fluoride (ArF) excimer laser (wavelength: 193 nm), which enables a line width of 100 nm or less, is expected to come into practical use. Furthermore, various kinds of radiation lithography technologies using a short wavelength, such as fluorine dimer (F2) excimer laser light (wavelength: 157 nm), extreme-UV rays, X-rays or electron beams, are in a developmental stage.
In semiconductor lithography processes, in a resist film where by making use of a variation of the solubility to an alkali developer under an action of acid, a resist pattern for transferring on a substrate is formed, and in an upper layer of the resist film or a lower layer of the resist film, various kinds of coating films are used. It can be cited that as a coating film applied to, for instance, a lower layer, that is, as a lower layer film, an anti-reflective film that suppresses light from being reflected from a substrate to accurately form a fine resist pattern, a flattening film that is used in a lower layer of a resist to make irregularity formed on a surface of the substrate flat when a resist pattern is further formed on the substrate thereon a pattern is formed, and an underlayer film or the like in a multi-layered resist that is used to transfer a resist pattern owing to dry etching.
These coating films can be formed in such a manner that a coating solution where a copolymer for lithography, which has a function of each of the coating films, other additives are dissolved in an organic solvent is prepared, the coating solution is coated on a substrate according to a method such as a spin coating method, and as needs arise the solvent is removed by heating or the like. In a copolymer used for lithography at that time, in addition to the optical properties that are demanded for a resist film and an anti-reflective film, chemical properties, coating properties and physical properties such as the adhesiveness to the substrate or the lower layer film, a fundamental property as a coating copolymer such that foreign matters that disturb the formation of a fine pattern are not present is demanded.
As a resist copolymer that is a copolymer used in a resist film, there are a negative type copolymer in which owing to an action of acid the solubility to an alkali developer is decreased and a positive type copolymer in which owing to an action of acid the solubility to an alkali developer is increased. The positive type resist copolymer is constituted necessarily including a repeating unit having a polar group that improves the adhesiveness to a semiconductor substrate and an underlayer film, or controls the solubility to a lithography organic solvent or an alkali developer and a repeating unit having a structure where a nonpolar substituent group is dissociated by acid to develop a polar group soluble in an alkali developer, and, as needs arise, a repeating unit having an acid-stable nonpolar substituent group for controlling the solubility to the lithography organic solvent and the alkali developer.
Specific examples of such a positive type resist copolymer include, in the KrF lithography process, a copolymer that includes a repeating unit derived from hydroxystyrene and a repeating unit derived from acid-decomposable alkoxystyrene; a copolymer that includes a repeating unit derived from hydroxystyrene and a repeating unit derived from an acid-decomposable alkyl (meth) acrylate; and a polymer wherein the hydroxystyrene-derived repeating unit has been partially protected with an acetal are known. In the ArF lithography process, a copolymer or the like that includes a repeating unit derived from (meth)acrylate substituted by a hydroxyalkyl group and a repeating unit derived from acid-decomposable alkyl (meth)acrylate is known.
A repeating unit that has a hydroxyl group is readily dissolved in an alkali developer. Accordingly, when the repeating unit that has a hydroxyl group is used as for instance a resist film, a resist pattern can be appropriately smoothed and the roughness can be suppressed low and so on. Furthermore, the solubility to the alkali developer is different depending on a molecular weight of a copolymer as well. That is, in general, the larger the molecular weight thereof is, the smaller the solubility thereof is, and the smaller the molecular weight thereof is, the larger the solubility thereof is. Accordingly, the higher a composition of a repeating unit that is small in the molecular weight and has the hydroxyl group is, the larger the solubility to the alkali developer is. In general, when a copolymer is designed, by taking such a nature into consideration, a composition of the repeating unit having a hydroxyl group and a molecular weight thereof are designed. Normally, a copolymer has a molecular weight distribution, and a composition of the polymer is different between a high molecular weight component and a low molecular weight component (hereinafter, referred to as “composition in a molecular weight direction”). Thus, when a composition of a hydroxyl group is different in a molecular weight direction, a pattern as designed cannot be depicted. For instance, when a repeating unit that contains a hydroxyl group in a low molecular weight region is contained much, a top shape of a pattern tends to be rounded, by contrast, when the repeating unit that contains a hydroxyl group in a low molecular weight region is contained less, the top shape of a pattern tends to be angulated or rougher. Such a problem is becoming incapable of neglecting as a pattern becomes finer.
As a copolymer of which composition in a molecular weight direction is controlled, an example that is a copolymer containing an alicyclic structure and a lactone structure, in which a lactone composition in a molecular weight direction is controlled within ±10%, is known (patent literature 1). Furthermore, an example that is a copolymer between a monomer having an alicyclic structure and p-acetoxystyrene, in which a p-acetoxystyrene composition in a molecular weight direction is controlled within ±10%, is known (patent literature 2). The technologies each have proposed a method where, in a two-component copolymer that has an alicyclic structure and a lactone structure or a monomer having an alicyclic structure and p-acetoxystyrene, in order to improve the solubility to a solvent, a lactone or p-acetoxystyrene composition is controlled.
As a similar technology, a technology where a monomer having a polar group such as a hydroxyl group and a monomer not containing a polar group are supplied in a heated polymerization solvent together with a polymerization initiator and a polymerization catalyst to polymerize is known (patent literatures 3 and 4). The technology has proposed, in order to improve the adhesiveness with a substrate, a method of polymerizing a monomer having a polar group-containing alicyclic functional group. However, in all of the above-described technologies, the relationship between a composition control in a low molecular weight region and the lithography characteristics is not disclosed.
In a polymerization solution after a polymerization reaction, other than a copolymer, there are low molecular weight impurities such as an unreacted monomer and impurities derived from the polymerization initiator or the polymerization catalyst. The low molecular weight impurities are unfavorable because, in the semiconductor lithography process, the impurities volatilize to stick to an optical system of an exposure device, generate a defect in a pattern or cause a variation in the nature of the copolymer during storage. In this connection, a method is known where a polymerization solution is mixed with a poor solvent to precipitate a copolymer as a solid content (hereinafter, referred to as “reprecipitation”) or a precipitated copolymer is washed with a poor solvent (hereinafter referred to as “washing”), and thereby the copolymer is refined (hereinafter referred to “refining”) owing to solubility difference to the poor solvent of the copolymer and the low molecular weight impurities. The refining process is applied in almost all of the above referenced literatures. Other than the above, a method where a composition of the solvent is controlled so that a residual monomer may be 5% or less (patent literature 5), a method where a copolymer-containing slurry dispersed in a solvent is heated (patent literatures 6 and 7) and a method where, by use of a poor solvent, reprecipitation or rinse is applied to remove an insoluble content to improve the solvent solubility (patent literature 8) or the like are known.
However, in all of the technologies, the poor solvent that is brought into contact with the copolymer is, in each of reprecipitation and/or washing step, only a polar solvent having a hydroxyl group or only a nonpolar solvent that does not have a hydroxyl group. In the refining step, since the solubility difference between a low molecular weight region of the copolymer and the low molecular weight impurities to be removed is small, the low molecular weight region of the copolymer is partially removed. Accordingly, there are problems in that, when a polar solvent having a hydroxyl group is used to refine, a composition of a hydroxyl group-containing repeating unit in the low molecular weight region of the polymer is lowered, and when a nonpolar solvent is used to refine, a composition of a hydroxyl group-containing repeating unit in the low molecular weight region of the polymer is raised.
From these backgrounds, in a copolymer for semiconductor lithography, which is obtained by copolymerizing a monomer having a hydroxyl group and a monomer that does not have a hydroxyl group, only a polymer where a composition of a hydroxyl group-containing repeating unit in the low molecular weight region is deviated from an average composition is known. Accordingly, a problem relating to a shape of a lithography pattern such as mentioned above is not yet overcome.
Patent literature 1: WO99/50322
Patent literature 2: JP-A 2001-151823
Patent literature 3: JP-A 2002-194029
Patent literature 4: JP-A 2003-306514
Patent literature 5: JP-A 2001-109153
Patent literature 6: JP-A 2002-201210
Patent literature 7: JP-A 2002-229220
Patent literature 8: JP-A 2003-213721